At one time, Pratchett declared that the Disc and Ankh-Morpork (two of the mapped regions) were unmappable.

„The reason I said it was unmappable was in those [other, traditional] fantasy books, the map was clearly drawn before the event. What we did was, after twelve, thirteen, fifteen books, then we mapped it. But the point is we mapped what was in the books. In other words, the landscape was created, then the landscape was mapped. We didn’t map and then, as it were, create the landscape based on the map. And indeed, the very act of mapping gave me fresh ideas and locations and the nature of the city of Ankh-Morpork.“